Test sockets adapted to carry integrated circuit (IC) chips and modules have been routinely used during final test and burn-in. These sockets are commonly used to hold in place a module carrying one or more IC chips, the combination of which is placed in an oven or a furnace during burn-in for a predetermined number of hours.
Burn-in is normally performed immediately after final testing. In as much as IC chips must function under varying environments and temperatures, burn-in is routinely appended to final testing to accelerate the occurrence of early fails (i.e., early life failures) in order to improve the overall product quality and weed out chips having reliability problems.
Sockets are designed to hold a variety of IC products, and are usually customized to the precise footprint of the chip carrier, (i.e., a single-chip module or a multi-chip module). Economic reasons encourage the reuse of a socket whenever possible even when ICs have different footprints and oftentimes different I/O contacts. Examples of the latter include pin-grid-arrays (PGA), land-grid-arrays (LGA), ball-grid-arrays (BGA), and the like.
Today's IC modules, particularly those made of ceramic, glass ceramic, or glass commonly encompass a plurality of layers, wherein each layer is used as a ground plane, a power plane, or as a personalization plane that provides interconnections between chips mounted on the module. The number of layers typically varies from 10 to 30. Practitioners of the art will fully appreciate that in order to reuse the same socket, it would be advantageous to have the socket accept IC modules of any thickness, as long as the variations are confined within certain acceptable bounds. The reusability of a socket is particularly important for CMOS products, where burn-in is of greater importance than, for instance, bipolar products, and which must be performed on a regular basis to ensure proper reliability of the product. Having to procure a personalized design that accounts for every type of module adds considerably to the cost of the product, and oftentimes makes the product uneconomical.
Another feature which must be taken in consideration while designing an IC module burn-in socket is its ability of leaving the IC chip exposed to the ambient of the chamber wherein a burn-in test takes place. Many designs are such that the IC chip or module are "buried" or sunk deep inside the base or are isolated from the environment by a cover. In such instances, the ability of the socket to dissipate heat to the ambient is limited; the high device to air thermal resistance will result in overheating of high power parts during test or burn-in.
A typical burn-in socket is described in U.S. Pat. No. 5,409,392 to Marks et al. It includes a base, a top, contacts, and movable latches. Additionally, the socket is provided with a stripper plate which serves as a table for supporting the IC to be tested. Typically, the socket is provided with a plurality of holes aligned with the engagement parts of the contacts. Whereas such a socket can be used for leaded or leadless ICs, its construction cannot be adapted to modules of varying thickness.
Another socket used for burn-in testing is described in U.S. Pat. No. 5,312,267 to Matsuoka et al., wherein a socket is provided with a pressure cover which can be opened and closed by way of a pivotal hinge. In a similar arrangement, disclosed in U.S. Pat. No. 4,456,318, to Shibata et al., an IC socket is provided with a base, a cover and a rotary lever coupled to a locking/unlocking mechanism which makes use of the sliding movement of the cover over the base on the basis of the rotation of the rotary lever. The IC is loaded after the "clam shell" style lid is opened. In both patents '267 and '318, the construction of such sockets precludes having the IC exposed to a free flow of air, by having the IC clamped deep inside the socket, which inhibits a forced convective cooling.
In yet another example of a state of the art socket, disclosed in U.S. Pat. No. 4,846,703, to Matsuoka et al., is an IC socket provided with a support that can be moved to an upper and lower position without having to apply undesired pressing forces. Sockets of this nature are designed only for lead frame IC and cannot be used, e.g., for land-grid-arrays. Moreover, in this patent, once the IC is loaded, electrical contact latches are retracted and then released over the leads of the IC. This makes an apparatus of this kind highly specialized and not readily adaptable to modules of varying thickness.
PGA, LGA, and BGA modules with high power dissipation and variations in burn-in and stress testing require that the thermal resistance between the chip or module under test and the chamber air be significantly reduced. This makes it possible to achieve shorter burn-in cycles at high temperatures and prevents thermal run-away on the more sensitive IC devices.
Thermal resistance of conventional sockets can be improved to a point by exposing the IC to free air flow. Further improvements can be obtained by having a conventional finned heatsink in direct contact with the device. Intimate contact must be maintained between the heat sink and the IC device. The height sand the planarity of the device relative to the substrate may not be consistent between one module and the next. Therefore, it is desirable that the heatsink be gimbled with respect to the socket cover.
Generally, each IC module is individually burned-in in its own socket which consists of a base containing electrical contacts and module alignment features. Sockets, typically, further include a cover that compresses the IC module or substrate onto the contacts, holding the various components together in place during testing. Such cover requires that there be sufficient mechanical advantage in the closure mechanism to allow an operator to manually close the socket. This manual action overcomes the force of the contacts that are being compressed under the IC module and facilitates the electrical contact to the test board. Furthermore, since the force that may be applied against a chip is restricted to no more than, typically, 30 psi, which is substantially less than the force required to compress the IC modules against the electrical contacts. The chip heatsink and module compression mechanism must remain independent of each other.
Prior art cover assemblies generally do not contain separate pressure plates and heatsinks. Accordingly, if an exposed IC chip is to be subjected to a burn-in test, danger exists that such an unprotected chip may be crushed by the combination pressure of the pressure plate and the heatsink. Furthermore, prior art cover assemblies are not designed to enable the heatsink and/or the chips to be exposed to forced air coming from the side of the socket assembly, an important requirement since, generally, a series of circuit boards, each containing a plurality of sockets are stacked in the burn-in chamber during testing.